Spike control for jet engines



United States Patent 3,131,534 SPiKE CGN'IEQL FOR JET ENGILNES John E. Taylor, East Cleveland, and Daniel P. Ross, Cleveland Heights, Gilli assignors to Thompson Raine Wooldridge Inc, a corporation of Ohio Filed Oct. 19, 1955, Ser. No. 541,346 3 laims. (61. oil-35.6)

The present invention relates to the control of jet type engines and is, more particularly, concerned with the provision of an improved control for the air inlet of such engines when operated at velocities above the speed of sound.

In recent years the development of jet type aircraft engines capable of moving through air at a velocity substantially greater than the velocity of sound has resulted in increased problems in the collection of air for engine operation. At velocities above the speed of sound, shock waves are initiated by the leading surfaces of any air collection device and it has been found necessary to provide air collection devices having a variable inlet geometry capable of modification to provide satisfactory inlet efiiciency over a wide range of aircraft velocities.

At the present time, one of the most successful types of air-inlets capable of operation at supersonic speeds comprises a substantially circular forward-facing opening with a centrally mounted slender spike capable of movement in a forward or rearward direction relative to the opening. The spike has a gradually tapered configuration and as a result, when it is moved axially in and out of the air inlet, the effective area of the inlet is modified and the position of the normal shock Wave, which occurs at the transition from supersonic to subsonic velocity, is modified. Since it is desirable, in order to provide maximum eliiciency, to position the normal shock wave in a particular predetermined point within the opening, ayially of thespike, and since the position of the normal shock will vary with the air inlet velocity, it has been considered essential to satisfatcory operation to provide accurate control means for changing the position of the spike during aircraft flight.

Various means have been proposed for accomplishing the above mentioned variation in spike position in response to changes in air inlet conditions. One of these proposed systems suggests the use of a radioactive material emitting alpha particles. This emitter was positioned immediately adjacent a receptor, both the emitter and receptor vbeingpcsitioned in the path of air flow within the air inlet, and the variation in current flow from the emitter to the receptor was note. Since increased current flows with an increased air density or pressure, under such an arrangement, the average position of the relatively high pressure normal shock wave along the relatively long, longitudinally extending, emitter could easily be found.

The present invention relates to an improvement in controls for an air inlet spike and utilizes a radioactive material capable of emitting beta particles rather than alpha particles. As those slclled in the art are aware, beta particles are of substantially less mass than alpha particles and are ejected from the radioactive substance with much greater velocity than alpha particles. For example, while alpha particles have velocities of the order of 2x10 centimeters per second, beta particles are ejected at velocities approaching the speed of light which is 3x10 centimeters per second. While alpha particles produce ionization of the air through which they pass at high speed, their movement through the air reduces their velocity so rapidly that effective ionization of only a small thickness of air is possible. Further, metallic coatings materially reduce the velocity of alpha particles and still further reduce the layer of ionized air surrounding ice an alpha emitter. In practice, therefore, the distance between an alpha emitter and a receptor, between which an electric current is to be passed through ionized air, is limited to a few centimeters. This limitation requires that apparatus project into the air stream when such a system is utilized.

in accordance with the present invention, all control functions are accomplished without the introduction of objects or irregular surfaces into the path of the air stream. It has been found that in view of the relatively light weight of the beta particles they are fully capable of ionization of gases, such as air, argon, methane, etc. used in conventional ionization chambers at distances at least as great as three feet from the source of beta emission. Further, it has been found that a thin metallic coating of stainless steel or the like may be placed over the beta emitting material without seriously effecting its ionization eihciency at these distances.

As will hereinafter more specifically be described, the present invention accordingly provides a beta emitter on the inner wall of an air inlet and houses an ionization chamber within the air inlet control spike itself, or vice versa. The electrical apparatus, of conventional nature, which is utilized for sensing the degree of ionization in the ionization chamber is likewise housed within the confines of the spike and as a result no obstructions whatever are positioned in the path of air flow to cause inefficiencies in the air inlet. Further, by withdrawing the pressure sensing apparatus from a position in the air flow, inaccuracies resulting from boundary layers set up by the apparatus projecting into the air flow, are eliminated. Additionally, a coating of stainless steel or the like may be provided over the beta emitting substance to protect personnel against excessive radiation and prevent injury to the material itself, without detrimentally affecting the operation of the engine.

It is, accordingly, an object of the present invention to provide an improved control for air inlets operating at supersonic air velocities.

Still a further object of the present invention is to provide an improved air inlet geometry control system capable of accurately modifying the inlet geometry without in any way affecting the efficiency of air fiow through the inlet.

Still a further object of the present invention is to rovide a beta ray apparatus for sensing pressure variations in an air inlet for supersonic engines or the like.

Another object of the invention is to provide an extremely sensitive control for air inlet ducts whereby variations in air pressure occasioned by variations in aircraft speed maybe instantly and accurately transformed into control of the air inlet.

A feature of the present invention is the utilization of beta emitting substances, such as for example strontium as a substance capable of accurately sensing pres sure change in aircraft air flow ducts.

Another feature of the invention resides in the utilization of beta rays emanating from a source flush with the walls of the aircraft air inlet, for controlling the air inlet configuration.

Still other and further objects and features of the present invention will at once become apparent to those skilled in the art from a consideration of the attached drawings wherein two preferred embodiments of the present invention are shown by way of illustration only and wherein:

FlGURE 1 is a side elevational View in partial cross section of an air inlet constructed in accordance'with the principles of the present invention;

FIGURE 2 is a side elevational view in partial cross tional manner.

section of a second modified form of the present invention; and

FIGURE 3 is an enlarged cross sectional view of a modified beta source having a continuous thin metal covering skin.

As shown on the drawings:

As described above, the present invention utilizes a beta ray source as a novel means for sensing the location of shock waves occasioned by movement of air or other compressible fluid at or above the velocity of sound. As shown in FIGURE 1 of the drawings, the conventional aircraft jet engine air inlet housing ill is provided having an opening generally indicated at 11. The opening 11 is ordinarily of substantially circular configuration and is provided with a longitudinally extending inlet area control spike 12 which may be moved axially along its axis relative to the lip 11a of the opening 11. In conventional practice, the control spike 12 is sharply pointed as at 13 and has a slender, tapered body of generally circular cross section. supersonic relative movement, air or other compressible liuid impinges upon the point 13 of the control spike 12 forming an initial bow wave, for example as shown at 1.4. As the air downstream of the bow wave 14 enters the inlet 11 diffusion through the speed of sound takes place at a normal shock wave 15 upstream of which the velocity of the air is above the speed of sound and is at a relatively low static pressure. Downstream of the normal shock wave 15, the velocity of the air is decreased to a value below the speed of sound and the static pressure is substantially increased above the pressure upstream of the normal shock wave.

It has been found that operation at supersonic inlet velocities requires extremely critical control of the air inlet to maintain elliciency thereof. Further, it has been found that relative to each inlet configuration selected, there is an optimum position for the normal shock wave.

.As velocity of the entering air varies, and as atmospheric conditions vary, the shock wave 15 will tend to move axially of the inlet 11, out of the above mentioned most efiicient position. By reciprocating the control spike 12 along its longitudinal axis, in and out of the opening 11, the annular dimensions of the inlet 11 are modified and as a result, the normal shock 15 will change its position. In general, as the velocity increases, it is necessary to move the control spike 12 outwardly of the opening in order to maintain the normal shock wave 15 in its upstream position generally shown in FIGURE 1.

Movement of the control spike 12 is preferably accomplished through an electrical motor of conventional form 16 mounted within a guide housing 17 which is in turn carried fixedly relative to the housing 16 on the bridge 18 of streamlined configuration. The armature of the motor 16 may be provided with internal threads such that rotation thereof will cooperate with an externally threaded actuating worm 1g which in turn cooperates with an internally threaded nut surface 26 rigidly associated with the spike 12. Thus, rotation of the motor 16 will cause reciprocation of the spike 12 which is supported and guided by the support 17, as at 17a, in a non-rota- It will, of course, be understood that various mechanical means may be utilized for causing reciprocation of the control spike 12 and any such means may be utilized in accordance with the present invention as long as it is capable of electrical control. For example, a pneumatic motor under the control of electrically actuated valves, or similar alternative structure may readily be used for applying reciprocatory motion to the spike 12.

In accordance with the principles of the present invention, the motor 16 is controlled by means of an electrical circuit responsive to variations in the electrical conductivity of a gas ionizable by bombardment with beta radiation. Extensive investigation has shown that radioactive materials of the beta radiation emitting type, such as for During supersonic flight, or other example strontium 90, which is a pure beta emitter, will readily ionize gasses conventionally used in ionization chambers, such as for example air, argon, methane and others considered conventional in the ionization chamber art. The degree of ionization varies substantially directly with the transmission of the beta rays and it has been found that the transmission of beta rays likewise varies inversely as the pressure in the medium through which the rays are traveling.

Further, it has been found that the small mass and extremely high velocities of beta particles permits the placement of the beta source a substantial distance from the ionization chamber. For example, it has been found that a spacing of three feet of air between a beta source, strontium 90, and an ionization chamber provides completely adequate ionization for the control of conventional electrical circuits throughout the range of normally encountered air pressures from 0 to 2 atmospheres. Since in structures under consideration, the distance from the surface of the control spike 12 to the inner surface 10b of the housing It is ordinarily substantially less than three feet it will be clear that positioning a source of beta radiation, such as strontium 90, at or near the surface of the spike 12, and positioning an ionization chamber at or near the surface 19b of the housing 10, or vice versa, will cause sufficient ionization in the ionization chamber for control purposes.

In FIGURE 1, an arrangement is shown wherein the beta source is provided in an annular layer 21 adjacent the forward edge 11a of the opening 11. An ionization chamber 22 is positioned within the control spike 12. As shown, the ionization chamber 22 is annular in shape in order to permit passage of the actuating screw 19 but it will be understood that with variations in actuating screw design, the chamber 22 may comprise a substantially cylindrical hollow chamber instead. In practice, the ionization chamber may be positioned as shown in FIGURE 1 and the surface of the spike 12 longitudinally slotted around its periphery to permit radiation from the source 21 to penetrate to the ionization chamber without undue attenuation. In such arrangement, it is preferred that a divider plate 22a be positioned in the ionization chamber compartment approximately at the point of impingement of the normal shock wave during operation at the design inlet air velocities. This dividing plate operates to prevent air flow back through the boundary layer adjacent the surface of the spike. Such flow would, of course, decrease the aerodynamic efiiciency of the diffuser and is accordingly undesirable.

An alternative to providing longitudinally extending peripheral slots in the spike 12, and a method which also eliminates all diificulties relative to undesirable effects on the air iiow past the spike 12, is the provision of a continuous, thin metal skin at the surface of the spike 12 and extending completely over the ionization chamber 22. In this alternative system slight attenuation, of course, takes place since the transmission of beta rays through any material is substantially the same per unit of mass (there being substantially the same number of electrons per unit of mass for all materials). Thus, since the mass of any metal used for the thin covering would be many times the mass of the same volume of air, the metal must be maintained at a very small thickness. This will be appreciated when it is recalled that the mass per unit of volume, or density, of air is approximately .081 pound per cubic foot as compared with steel which is on the order of 490 pounds per cubic foot. Thus, the resistance to' transmission of the beta rays is approximately 6050 times as great as that of air. It is preferred that the thickness of the metal covering the ionization chamber be accordingly maintained at a few thousandths of an inch. This arrangements is shown in FIGURE 3.

In view of the fact that the transmission of beta rays through metal is substantially lower than the transmission through air, it is also desired that the beta source itself plating on the inner surface of the annular beta source the thickness of the plating 21a which covers the beta source, this difference and distance may be compensated for. Thus, a very slightly heavier plating is preferably provided at the leading edge and gradually lessens in thickness as it proceeds toward the rear edge of the source.

in practice, it has been found that for use in diffusers of the type generally illustrated in FIGURES l and 2 of the drawing, in which the ionization chamber is relatively close to the source, the axial width of the ionization chamber may substantially equal the axial width of the source. This is true since even at operation at missile or aircraft velocities of approximately 5x10 centimeters per second, the air velocity isnegligible compared to the beta velocity which in the case of strontium 90 is substantially 2.16 X centimeters per second which is in turn well over two-thirds the 3 X 10 centimeters per second velocity of light. As a result, deflections of the beta rays as a result of the velocity of the air have substantially no effect on the system,-in spite of the relative ease with which beta rays are defiected'compared to alpha particles.

Since beta transmission varies substantially, and inversely to air pressure, the ionization resulting in the ionization chamber from the transmission of beta rays through air entering the inlet 11 will vary inversely with an average increase in pressure between the source and the chamber. Likewise, the electrical conductivity of the ionization chamber will decrease with the reduction of ionization accompanying such as pressure increase. Since the pressure of the air downstream of the normal shock wave is substantially greater than the pressure upstream of the wave, sharp variationsin average pressure will occur between the total surfaces of the source and the ionization chamber as the shock wave moves upstream or downstream. Thus, when the normal shock wave is positioned at the upstream edge of the source 21 a maximum pressure exists between the source and the ionization chamber, and accordingly, a minimum ionization occurs in the ionization chamber, with a minimum'current flow. Likewise, with the shock wave positioned at the trailing edge of the source 21, a minimum average pressure will prevail and a maximum ionization, with maximum current how, will occur. A conventional bridge circuit may thus be utilized to actuate the motor is; either in a forward or reverse direction upon fluctuation of the electrical conductivity of the ionization chamber above and below the conductivity, and hence the beta current flow, at the average value which occurs when the shock wave is positioned at some intermediate, maximum efiiciency, axial position relative to the source'21.

in order to provide automatic compensation of the pressure sensing system thus provided, a reference gauge is prefe ably provided. This may take the form of a second beta source 23 which emits rays acting on a second ionization chamber 24. This second ionization chamber is provided with an electrical connection similar to that of the primary ionization chamber 22 and provides a varying current flow which decreases with an increase in air pressure between the source 23 and the chamber 24. The secondary source 23 and chamber 24- provides a standard current flow which will reflect changes in the transmission of the beta rays resulting from temperature changes, air density variations resulting from altitude changes, and the like. By incorporating the ionization chamber 24 into the electrical bridge circuit, along with an adjustable current flow controlling rheostat as one leg of a conventional bridge and in parallel with the ionization chamber 22, the bridge circuit may be balanced to provide no current fiow to the motor 16, in either direction, when the normal shock wave 15 is at a predetermined optimum position. Variation of the shock wave fore and aft of this predetermined position will cause a variation in the current How in that portion of the bridge circuit containing the ionization chamber 22, thereby unbalancing the bridge and causing a current flow to the motor 16 to move the spike in a direction to return the normal shock wave to its predetermined desired position. Thus, if the normal shock wave 15 moves downstream With a resultant increase in current flow across the ionization chamber 22, the bridge circuit will be unbalanced to cause a current flow through the motor 16 to rotate the actuating screw 19 to move the spike 12 to the right as viewed in FIGURE 1, thereby causing the normal shock wave 15 to occur upstream in its proper position.

As may be seen from a consideration of FIGURE 1, the present system permits an extremely compact arrangement which in no way interferes with efiicient air flow. Thus, neither the primary nor secondary beta sources 21, 23 respectively, or their respective ionization chambers 22, 24, project in any way into the air stream. Further, by positioning the ionization chambers in the spike 12, the electrical conductors 22b and 24!) associated with the respective chambers 22 and 24 pass directly to the central control unit 25 in which the bridge circuit connections are made, without crossing the path of air flow. Likewise, the motor control leads as are housed within the main central support 17. The only controls that must be brought to the control center 25 through one of the bridge supports 13 are the conductors 27 which provide motive power for the electric motor 16. If desired, the conductors 27 may include remote control for theabove mentioned bridge balancing rheostat to provide for optimum adjustment of the normal shock wave position.

As noted above, the position or the primary and se ondary beta sources may be reversed relative to their respective ionization chambers, thereby placing the source of beta emanation in the control spike l2 and theiionization chambers on the inside surface of the housing 10. This arrangement is illustrated at FIGURE 2 wherein the beta source, for example strontium 90,-is. illustrated as comprising a coating 239 on the spike 12 while the ionization chamber comprises an annular enclosed-chamber 22% positioned at the interior surface of the housing 10. Likewise, in the case of the secondary reference system, the beta emitter comprises coating 22% on the surface of the spike i2 and the ionization chamber 24% positioned on the interior surface of the housing It downstream of the ionization chamber 226. As will be'noted, the conductors 22th; and 24% leading to the control cen ter 25 rnust'of necessity be substantially longer and pass through the housing The control conductors 27 which supply the motive power to the electric motor 16 are, as in the case of the embodiment shown in FiGU-R'E 1, passed through one of the supporting bridge members the separate species illustrated in FIGURES l and 2 will be substantially the same;

In addition to the structure shown in FIGURES 1 and 2, it will, of course, be understood that under some circumstances it may be found desirable to provide a longitudinally extending strip of beta ray source, either on the spike 12 or the inside surface of the housing It), with a corresponding longitudinally extending ionization It will be apparent, of course, that the operation of chamber, rather than the annular sources and chambers above described. Such an arrangement would, of course, provide less radiation and, accordingly, a smaller control current in the ionization chamber circuit. This deficiency may be immaterial, however, in some uses, such as for example where the air inlet is small and the distance from the source to the ionization chamber is reduced to a point Where attenuation resulting from trans mission of the beta rays through the air is small enough to justify a smaller source. In addition to such variations in physical construction it will also be understood that the present invention is not limited to the use of such beta emitting substances as strontium 90. Strontium 90 has been specifically mentioned since it is relatively inexpensive and has a half life of approximately 25 years thereby rendering replacement unnecessary. However, substantially any beta emitting substance may be utilized and the exact substance from which the beta emanations come is not considered critical in the present invention. For example thallium-204 having a half life of 2.7 years is suitable although its cost is several times that of strontium 90 on the present market.

It will thus be understood that we have provided a substantially improved system for sensing the position of a shock wave within an air inlet. In accordance with the principles of the present invention absolutely no obstructions are presented to the internal flow of air in the diffuser or the like with which the sensing apparatus is utilized and errors ordinarily resulting from turbulence caused by obstructions presented by conventional apparatus are eliminated. Further, an unusually sensitive sensing system has been herein set forth since it has been found that beta transmission through air is extremely sensitive to variations in air pressure.

While we have described the apparatus of the present invention in connection with a spike controlled diffuser opening, it will be understood that the sensing apparatus of the invention may be used equally well with other types of variable geometry openings. For example, the position of the shock wave in the diffuser may be controlled by opening air bleed doors in the diffuser wall aft of the normal shock wave position or by varying the position of deflector flaps pivoted to the spike upstream of the normal shock wave position. Additionally, in diffusers of the type mounted at the side of the aircraft fuselage and which use no spike, a wedge type ramp which controls the diffuser opening may be angularly varied to control the shock wave position. In these variations, of course, the sensing means above described is connected to change the doors, flaps, or ramps to vary the shock wave pattern as flight conditions change.

Since it will be obvious to those skilled in the art that variations and modifications may be made in accordance with the principles of the above invention Without departing from the scope of the novel concepts thereof, it is our intention that the scope of the invention be limited solely by that of the hereinafter appended claims.

We claim as our invention:

1. In combination in a jet engine for aircraft constructed to travel at velocities in excess of the velocity of sound, a housing member having an air inlet at one end adapted to receive air at a supersonic velocity and a smooth unbroken internal surface leading axially downstream from said inlet, a reciprocably movable control spike member mounted generally centrally of said inlet and having a streamlined smooth unbroken external surface facing said housing internal surface, means for reciprocating said spike to position said spike at an optimum position in which the shock wave caused by reduction in velocity of said air to a subsonic velocity is positioned within said inlet at a predetermined axial position relative to said surfaces, said means including means for sensing the position of said shock wave without substantially afiecting air flow conditions, said last named means comprising a beta emitting substance extending a substantial axial distance in and along the sunface of one of said members upstream and downstream of the desired position of the shock wave and providing a smooth unbroken surface with said surface of one of said members and a substantially axially coextensive ionization chamber placed under the surface of the other of said members substantially opposite from said emitting substance, means passing an electrical current through said ionization chamber and means responsive to changes in current flow through said ionization chamber resulting from a change in the average air flow pressure therebetween caused by axial movement of said shock wave, to reciprocate said control spike to return the shock wave axially to its desired position.

7 2. In combination in a jet engine operative at air inlet speeds in excess of the velocity of sound, a housing memher having an air inlet at one end thereof adapted to receive air at a supersonic velocity and having a substantially smooth internal surface leading axially down stream from said inlet, a reciprocably movable control spike member mounted generally centrally of said inlet and having a streamlined substantially smooth external surface facing said housing internal surface, means for reciprocating said spike axially of the air stream to position said spike at an optimum position in which the shock wave caused by reduction in velocity of said air from supersonic velocity to a subsonic velocity is positioned within said inlet at a predetermined axial position relative to said internal surface, said means including means for sensing the position of said shock Wave without substantially affecting air tiow conditions, said last named means comprising a beta emitting substance extending a substantial axial distance in and along the surface of one of said members both upstream and downstream of the desired position of said shock wave and a substantially axially coextensive ionization chamber placed smoothly in the surface of the other of said members substantially opposite from said emitting substance, means passing an electrical current through said ionization chamber, and means responsive to changes in current flow through said ionization chamber in response to changes in the average pressure of the air flowing between the emitter and chamber to reciprocate said control spike to maintain the spike in the position in which the shock wave is in its predetermined axial position.

3. In combination in an aircraft jet engine for operation at velocities in excess of velocity of sound, a housing -rnember having an air inlet at one end thereof adapted to receive air flow at a supersonic velocity and having a substantially smooth internal surface leading axially down stream from said inlet, a reciprocably movable control spike member mounted generally centrally of said inlet and having a streamlined substantially smooth internal surface facing said housing internal surface, means for reciprocating said spike to position said spike at an optimum position in which the shock wave caused by reduction in velocity of said air from supersonic to a subsonic velocity is positioned with said inlet at a predetermined axial position relative to said internal surface, said means including means for sensing the position of said shock wave without substantially affecting air flow conditions,

said last named means comprising a beta emitting substance smoothly embedded in one of said surfaces and a first generally axially coextensive ionization chamber placed in the other of said surfaces substantially opposite from said emitting substance, means passing an electrical current through said first ionization chamber, said current flow decreasing as said shock wave moves upstream of said air flow relative to the beta emitting substance, and reference means for compensating for air flow conditions other than shock wave position, comprising a second beta emitting substance in said one surface downstream of the shock wave and the means for sensing the posi tion of said shock wave and a second ionization chamber 10 in the said other surface down stream of the shock wave References Cited in the file of this patent and a said means for sensing the position of said shock UNITED STATES PATENTS Wave, means passing an electrical current through such second ionization chamber and means responsive to 2,638,738 Salter, 1953 2,739,283 Roehng Mar. 20, 1956 changes in current flow through second ionization cham- 5 her to modify the changes in current flow through the first ionization chamber of said means for sensing the FOREIGN PATENTS position of said shock wave. 709,300 Great Britain May 19, 1954 

1. IN COMBINATION IN A JET ENGINE FOR AIRCRAFT CONSTRUCTED TO TRAVEL AT VELOCITIES IN EXCESS OF THE VELOCITY OF SOUND, A HOUSING MEMBER HAVING AN AIR INLET AT ONE END ADAPTED TO RECEIVE AIR AT A SUPERSONIC VELOCITY AND A SMOOTH UNBROKEN INTERNAL SURFACE LEADING AXIALLY DOWNSTREAM FROM SAID INLET, A RECIPROCABLY MOVABLE CONTROL SPIKE MEMBER MOUNTED GENERALLY CENTRALLY OF SAID INLET AND HAVING A STREAMLINED SMOOTH UNBROKEN EXTERNAL SURFACE FACING SAID HOUSING INTERNAL SURFACE, MEANS FOR RECIPROCATING SAID SPIKE TO POSITION SAID SPIKE AT AN OPTIMUM POSITION IN WHICH THE SHOCK WAVE CAUSED BY REDUCTION IN VELOCITY OF SAID AIR TO A SUBSONIC VELOCITY IS POSITIONED WITHIN SAID INLET AT A PREDETERMINED AXIAL POSITION RELATIVE TO SAID SURFACES, SAID MEANS INCLUDING MEANS FOR SENSING THE POSITION OF SAID SHOCK WAVE WITHOUT SUBSTANTIALLY AFFECTING AIR FLOW CONDITIONS, SAID LAST NAMED MEANS COMPRISING A BETA EMITTING SUBSTANCE EXTENDING A SUBSTANTIAL AXIAL DISTANCE IN AND ALONG THE SURFACE OF ONE SAID MEMBERS UPSTREAM AND DOWNSTREAM OF THE DESIRED POSITION OF THE SHOCK WAVE AND PROVIDING A SMOOTH UNBROKEN SURFACE WITH SAID SURFACE OF ONE OF SAID MEMBERS AND A SUBSTANTIALLY AXIALLY COEXTENSIVE IONIZATION CHAMBER PLACED UNDER THE SURFACE OF THE OTHER OF SAID MEMBERS SUBSTANTIALLY OPPOSITE FROM SAID EMITTING SUBSTANCE, MEANS PASSING AN ELECTRICAL CURRENT THROUGH SAID IONIZATION CHAMBER AND MEANS RESPONSIVE TO CHANGES IN CURRENT FLOW THROUGH SAID IONIZATION CHAMBER RESULTING FROM A CHANGE IN THE AVERAGE AIR FLOW PRESSURE THEREBETWEEN CAUSED BY AXIAL MOVEMENT OF SAID SHOCK WAVE, TO RECIPROCATE SAID CONTROL SPIKE TO RETURN THE SHOCK WAVE AXIALLY TO ITS DESIRED POSITION. 